Intrinsic ferromagnetism and restrictive thermodynamic stability in MA2N4 and Janus VSiGeN4 monolayers

The seminal experimental discovery of the remarkably stable $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the ${\mathrm{MA}}_{2}{\mathrm{Z}}_{4}$ family (M = transition metals, A = Si, Ge, and Z = N, P, As). These magnetic monolayers were predicted to be dynamically stable, but none of them has been synthesized to date. In this Research Letter, from first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable and this occurs under N-rich conditions. Based on this finding, we propose two ferromagnetic, semiconducting Janus monolayers in the family: ${\mathrm{VSiGeN}}_{4}$ and ${\mathrm{VSiSnN}}_{4}$. They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, Janus ${\mathrm{VSiGeN}}_{4}$ and ${\mathrm{VSiSnN}}_{4}$ monolayers show weak in-plane anisotropy compared with the $\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayer. These two emerging Janus magnetic semiconductors offer opportunities for studying 2D magnetism and spin control for spintronics applications.